# Current transformer distortion

• posted

Hello.

I am trying to develop a suitable current transformer to measure currents from 0 to 2A rms. Things are not going very well.

To start with, let me explain I tried with different ratios and no matter what I do, I end up with the same somewhat distorted shape (tough different amplitudes).

Test setup is: a 50Hz/45VA transformer with 10 Vrms output (note clipped sinewave), which is connected to a power resistor. The current flows through the power resistor into the primary of the CT (ratio is

2:60), the CT secondary is terminated by a 0.33 ohm resistor and the voltage across it is amplified by a factor of 100 with a differential opamp.

Pictures can be viewed here:

Questions:

Why the ratio error?

When I measured the frequency response of the CT I got surprising results. First: measuring the secondary inductance on a RCL meter resulted in inductance X. But when measuring the inductance from a few Hz to about 100kHz shows an inductance of X/2. Why?

Another problem is the resistance of the secondary, which is on average say half of the secondary reactance @50Hz.

Now suppose I want to have two indipendent secondaries on my CT, both terminated with a 0.33 ohm burden. How would this effect my "new" CT?

• posted

• Cos you have no clue what a CT eve n is.
• The AC supply voltage IS " somewhat" distorted - FUCKWIT !!
• So you have a 240volt to 10 volt 45VA tranny ??

Makes the ratio about 22 to one - pal.

• Purest gobbledegook.

Which planet are you from ???

In which universe ??

• Errrr - why ??
• Shows typical mains voltage waveform with varying amounts of phase shift.
0.33 ohms loading a 10 volt, 4.5 amp secondary will do that.
• Err - maybe you are a total fuckwit ??

• I could tell you, but that would spoil a very funny story.
• Noooooooooo ???

Fancy that.

...... Phil

>
• posted

Thank you for your comment Phil. I come from the planet Earth, where

• some* people can actually read and few of them bother to understand what they read. Now let us leave room, for someone who is not completely retarded, to respond...
• posted

As some others have pointed out, the oP was fundamentally gobbledy-gook. Instead of trying to figure out what you are saying, look at what you need to make a good current transformer.

Low leakage reactance. As you get less, any nonlinearity affects distortion less.

Hig magnetization inductance. You want excess ampere-turns. That is you want to work far away from saturation. You wanty a high permeability core that woks well across the frequencies whose harmonics you wish to include.

You say nothing I can understand about leakage inductance, permeability, ampere-turns, eddy currents, and almost aything else that would be useful.

You are the engineer whether qualified or not.

Bill

• posted

Thank you for your comment Phil.

• Which head is doing the thanking ??

I come from the planet Earth,

• A likely story....

where

• some* people can actually read and few of them bother to understand what they read.

• That leaves out all twin-headed fuckwits like you - pal.

Now let us leave room, for someone who is not completely retarded, to respond...

• Only the completely retarded would reply seriously to such a pile of putrid crapology.

Piss off - you damn GOOGLE TROLL !!

..... Phil

• posted

Since your post does not make much sense, lets do this one part at a time. Why is it clipped? If you are not saturating the transformer it should not be clipped.

Charles Perry P.E.

• posted

"Charles Perry"

• It is not " clipped " !!!

Just the usual AC supply voltage wave with flat tops.

Then phase shifted versions due to the OP's hilarious shenanigans with some innocent transformer.

.... Phil

• posted

To clarify I have uploaded three more pictures. Order is: 45Vac secondary voltage, CT burden voltage, test setup.

Now to clear things up, the discussion is not about the 45VA transformer. There is nothing wrong with it, it is merely used as a power source to provide the measured current. OK?

I am missing a post, so I will rewrite what was measured in the CT:

Secondary resistance: 2.57 ohm Secondary inductance: 20mH with RCL, 10mH measured by hand.

Primary resistance: estimated to be about 3 mOhm. Primary inductance: 1uH

Core is an unknown ferrite, no gap.

Opamp has been veryfied to work correctly at all times. I tried the same ferrite core with different winding ratios, the secondary varying from 15 to 120 spires, primary always had 2 spires. Waveform looks the same at all time, except for the amplitude value and the value of the current at which saturation starts to show (for different secondaries raging from 1.2 to 2.7A).

@Charles Perry: are you talking about the CT or the 45VA transformer?

@Phil: you are no longer welcomed with your comments in this thread as you clearly do not have anything to offer except trolling. Bye.

• posted

One problem is that the pictures are unlabeled as to which is which. In addition, it woulld help if the pictures indicated the magnitudes involved - something that you could do as you interpret the scope data. From the pictures, I have no idea of the quantities and magnitudes being measured. There is, as you indicate, distortion of the input. It does not necessarily follow that the CT secondary current will have the same amount of distortion. Salmon Egg has indicated some related factors .

There are too many unknowns to allow a reasonable answer.

As to inductance measurement- I assume that the measurement with the RCL meter doesn't include the source but is a measurement at the CT secondary with either open circuit or short circuit at the primary, while the other measurement implies a source and measurement based on some unspecified factors.

You cannot measure the reactance of the secondary alone.

In other words- tell us the conditions under which you made measurements? I can make guesses but these would be meaningless on the basis of the unknowns (e.g. If a measurement with the primary open indicates a reactance which is only twice that measured otherwise- you have a very poor CT.) Chances are that saturation is not the problem.

Salmon Egg has brought up some good points with respect to current transformers. Phil hasn't, in his rush to abuse, hasn't.

The crux of the matter is that you have actually not given the pertinent information to answer your questions. Have you run open and short circuit tests on the CT itself to determine its parameters?

• posted

"Don Kelly"

• Wasting his time of fuckwit Google Gropers like:

snipped-for-privacy@gmail.com

• Correct.

So don't even try.

• Bull.
• Cos Phil is smarter that either of you.

He can pick a retarded, cretinous Google Groups time waster miles off.

• And the cretinous PITA never will.

..... Phil

• posted

It sounds to me like you have a primary circuit consisting of a 10 V power supply, a power resistor and the primary of your transformer all connected in series. Is this correct?

You don't mention the value of the 'power resistor', but if we go by your desire to measure up to 2A of current, then we can assume it's a 5 ohm resistor. At 2A load, that resistor would have to be rated for 20 watts or more (more would be better for margin).

You say the transformer is terminated by 0.33 ohm and has a ratio of 2:60. So at that ratio, the reflected impedance of the terminating resistor into the primary would be 0.33/(30^2) = 0.367 milliOhms.

So one obvious check, that the reflected impedance will not significantly change the primary circuit is not the problem. If the resistor connected across the secondary of a CT is very high, then it can cause problems.

You don't supply much information about the CT, is it hand-wound or something? What does it have for a core? Saturation of the core can be a problem, but the waveforms don't really seem to show that sort of problem.

I don't know which of your graphs are what, so I can't tell what each one is displaying. Are these all just shots of the CT terminating resistor or what?

The obvious things to look at are:

1) Does your 10 V power supply have the same distortion? 2) Does the voltage across the 'power resistor' in the primary show this distortion? 3) Is the distortion coming from your op-amp and not really the CT?

Measurements of transformers is trickier than just an RCL meter. You might get the leakage reactance that way, but the interaction of the two windings in operation makes the only practical way to take these measurements is in operation.

Also if the resistance of the winding is an appreciable fraction of the transformer's characturistic impedance, it will introduce some phase shift.

Badly. The currents in the two secondaries would not be proportional to the primary current alone. I think that variations in the two secondary terminating impedances would cause the distribution of currents between the two to be hard to predict. Unless you're just going to add the two numbers together anyway, it seems to me the individual measurements would be pretty meaningless. And if you're just going to do that, why bother with two secondaries in the first place.

daestrom

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